In a hydraulic apparatus in which a load such as a hydraulic motor is driven by a hydraulic pressure generated by a hydraulic pump of fixed discharge amount type, for example, the amount of operating fluid discharged from the hydraulic pump is fixed. Therefore, when the amount of operating fluid needed by the load fluctuates, an excess of operating fluid occurs. Therefore, means for changing the number of revolutions of the hydraulic pump and means for adjusting the flow rate by flow rate adjusting means such as a throttle valve or reducing valve have been employed in general in order to supply an amount of operating fluid required by the load.
However, each of the hydraulic pump and the driving source such as a heat engine or electric motor for driving the hydraulic pump is hard to keep a high efficiency in all the revolution ranges, whereby the efficiency in the hydraulic apparatus may deteriorate when the hydraulic pump changes its number of revolutions. Also, the flow rate adjustment consumes the hydraulic energy as thermal energy, which may lower the efficiency in the hydraulic apparatus.
Hydraulic apparatus using a variable discharge type pump for supplying a necessary amount of operating fluid to the load have also been known. However, such a pump has a complicated structure and is expensive.
Therefore, it is an object of the present invention to provide a hydraulic apparatus which can efficiently supply a load with an operating fluid within the range from a low flow rate to a high flow rate while keeping a substantially fixed amount of discharge from a hydraulic pump.